1. Technical Field
This invention pertains broadly to the field of heat exchangers, and more specifically to arrangements of components in large intercoolers or aftercoolers.
2. Prior Art
Large compressors often use heat exchangers as aftercoolers or as intercoolers between compression stages. The horsepower required for achieving the desired final pressure is related to the temperature and pressure of the fluid being compressed. The horsepower required increases if the temperature increases or the pressure decreases at any compression stage inlet. An intercooler should, therefore, effect a significant temperature change at minimal pressure loss. Typically, the heat exchangers have shells with inlets and outlets for the process fluid and have heat transfer surfaces such as tube bundles or finned tube coils within the shells through which the conditioning fluid flows. The process fluid flows from the inlet in the shell along the heat transfer surfaces in heat exchange relationship with the conditioning fluid and flows out through the outlet of the shell. In many such systems, large quantities of condensate are formed as the process fluid is cooled, and, due to high fluid velocities, much of the condensate becomes entrained in the fluid flow. Disentrainment of liquids can be a problem in intercooler systems of compressor plants. The accepted practice has been to use woven wire, chevron separators or cyclone separators downstream of the heat exchanger. An undesirable pressure drop is experienced in such separators, which can add significantly to the compressor plant operating costs, and the separators also add significantly to the overall capital cost and size of the intercooler system.
Intercooler heat exchangers often are very large; however, because the heat exchanger must function within the overall system which may include several stages of compression and several intercoolers, often relatively limited space is available for each heat exchanger. Thus, achieving the necessary heat transfer and moisture disentrainment within the space available can be difficult. The design of such a heat exchanger is further complicated by limitations in the suitable locations for the inlet and outlet nozzles of the heat exchanger which must connect with other system components, and by the velocity of flow of the process fluid. A designer of an intercooler is faced, therefore, with many fixed requirements, including the maximum shell size, the location and spacing of nozzles, the size of the piping to the nozzles and the maximum pressure drop allowable in the intercooler. These limitations make it difficult to achieve the objective of maximum temperature reduction at minimal pressure drop.
Frequently, design requirements are for inlet and outlet nozzles of the shell to be in close proximity. Maldistribution of the fluid then becomes a problem, with most of the fluid flowing along the area of the tube bundle or coil near the nozzles. Only a minimal flow occurs through portions of the heat exchanger on the side of the inlet opposite the outlet, with the portions farthest therefrom experiencing the least flow. This maldistribution problem makes proper sizing and heat exchange calculation very difficult. High shell side velocities can result in further maldistribution problems, contributing to higher pressure drops and decreasing the heat transfer performance of the heat exchanger. If shell side velocities can be reduced, maldistribution is lessened. In the past this has been difficult in that the vehocities to and from the heat exchanger are fixed by the requirements of the system. It would be beneficial, therefore, to reduce velocities within the shell.
Another problem encountered in the design of a compressor intercooler concerns servicing the intercooler and especially the tube bundles therein. Periodic cleaning and inspection of the tubes is desirable, and access to the tube bundles should not be difficult. In some instances, it is desirable to inspect and clean the tube bundles in a relatively short period of time, leaving the coils in place and without having to disconnect the water piping to the coils. In other instances it is desirable to remove the coils from the shell. In either case, easy access to both sides of the coil should be available for cleaning all fin surfaces. It is also desirable to be able to replace only a portion of the cooling coil, if necessary, and to be able to do so quickly. In previous designs for such heat exchangers, if part of the coil needs replacing, a substantial portion or all of the coil had to be replaced.